In persons with osteoporosis, vertebral fractures occur in approximately 750,000 reported cases each year in the United States alone. These fractures can cause acute or chronic pain, reduce the quality of life, and shorten life expectancy (N. B. Watts, Osteoporotic Vertebral Fractures, Neurosurg. Focus. 2001 Apr. 15; 10(4): E12). Percutaneous vertebroplasty (“PVP”) and percutaneous kyphoplasty (“PKP”) are minimally invasive options used to treat vertebral fractures and osteolysis.
PVP involves stabilizing a fracture by injecting a bone cement into the inter-trabecular marrow space of a fractured vertebra, usually under fluoroscopic or other radiological guidance. In PKP a balloon is inserted into the inter-trabecular marrow space of the damaged vertebra, which is then inflated to restore the fractured vertebra as much as possible to its original shape. The balloon is then deflated and removed, and the cavity formed is filled with bone cement. In both methods the injected bone cement functions as a supportive material that to some extent fills voids and stabilizes the vertebral body.
The most commonly used bone cements include polymethyl methacrylate (PMMA). These cements are prepared by mixing methyl methacrylate powder and a radiopaque agent with liquid monomer. Once mixed, the bone cement is injected under radiological imaging into the vertebral body where it cures in situ into a hardened material. Although widely used, PMMA bone cements have a number of disadvantages including one or more of the following:
(1) They are hard and stiff and are suspected of contributing to adjacent level fractures in other vertebrae following PVP or PKP.
(2) Because of their hardness, they may exert pressure on nerves and tissue causing pain and/or nerve or tissue damage.
(3) Once mixed the bone cement is often very thick, difficult to inject into the body and does not completely fill the spaces into which it is injected.
(4) The mixing of the cement components is done by a doctor or other person (hereafter, “user”) at the location (such as a hospital) where PVP or PKP is performed. The mixing is susceptible to error, particularly (a) the addition of excess monomer, which is toxic and may cause adverse reactions (such as pulmonary embolism or hypotensive reaction that can cause respiratory or cardiac failure, which can lead to death), or (b) the addition of too little monomer, which will cause the cement to be thick, to harden too quickly, and make it difficult to inject into the body.
(5) The mixing of the PMMA cement components can be uncomfortable for the user due to strong, unpleasant odors generated during the process. Some users have had to incur the expense and burden of installing exhausting vent hoods within their operating rooms to deal with this problem.
(6) The in situ curing of the cement is exothermic, which can lead to necrosis of the surrounding tissue thereby harming a subject being treated.
(7) PMMA bone cements usually are porous and can absorb water, which leads to degradation of the bone cement over time.
(8) Once injected into the vertebral body, PMMA bone cements tend to form lumps of material within the vertebral body rather than evenly filling the space in the vertebral body.
(9) PMMA bone cement does not adhere well to bone.
(10) PMMA bone cement tends to physically displace the trabeculae during injection and curing.
(11) PMMA bone cement does not allow the user to easily stop and start the injection procedure once it has begun.
Thus, PVP and PKP would benefit from an injectable composition of biomaterial that does not have the problems associated with the use of PMMA bone cements or bone cements with problems similar to those of PMMA bone cements. Another optional benefit would be obtained if the device used to mix and/or inject the composition of biomaterial eliminated the requirement of the user having to mix the components that form the composition.